The Oldam-ring of the rotational force transmitting mechanism of a fluid compressor is engaged with the rectangular section formed on the base portion of the main shaft of the rotary rod in the direction perpendicular to the axial direction of the main shaft to achieve an easy assembling of the Oldam-ring to the rectangular section. If the fluid compressor employs a thrust force canceling system wherein the thrust force acting on the rotary rod from the discharge side toward the suction side of a cylinder is canceled by a counter thrust force, the diameter of the main shaft located at the suction side is increased compared with that of the conventional fluid compressor to increase the counter thrust force acting on the edge surface of the main shaft whereby the thrust force can be effectively canceled.
